An oblong building with an apse at one end and lighted from above. It was usually rectangular with a width not greater than one-half nor less than one-third, its length, divided by rows of columns into a central nave and a surrounding lower, narrower aisle or ambulatory. The upper part of the nave (clerestory) was lighted by a row of arched window over the roofs of the adjoining aisles, and similar windows lighted the aisles. Basilicas were the first pagan edifices to be converted into Christian churches, being best adapted for Christian worship. The altar was placed within or before the apse, and arches from nave, aisles, and apse opened into the transept, a cross hall of the same height as the nave interposed between nave and apse for practical purposes and for the symbolism of the cross. At the entrance end opposite the apse was the narthex, a portico beyond which neophytes were not at first admitted. As the priest was supposed always to face the east, basilicas were built with the entrance facade toward the east when he faced the congregation and toward the west when it became customary for him to turn his back to them.

The title of basilica is now given by the pope to privileged churches remarkable for antiquity or historical associations. They are either major (patriarchal) or minor, privileged with the right of precedence as churches, special insignia, and a college of clergy entitled to the rochet and cappa. Among the most notable are those of Saint Peter, Saint John Lateran, and Saint Mary Major, Rome, and Saint Francis Assisi.